@rust-timer queue should not modify unrelated labels

[jyn514]

There are currently 7 open PRs marked both S-waiting-on-author and S-waiting-on-review in rust-lang/rust: https://github.com/rust-lang/rust/issues?q=is%3Aopen+label%3AS-waiting-on-author+label%3AS-waiting-on-review

This is an invalid state and confuses reviewers, authors, and triagers. Nearly all of them are because rustbot adds a waiting-on-review label when it finishes running: rust-lang/rust#104898 (comment)

It shouldn't do that. It should only add waiting-on-perf when it starts and remove waiting-on-perf when it finishes.